Coated orthopaedic implant components

ABSTRACT

An improved coated orthopaedic implant component is disclosed. The implant may be coated with platinum, iridium or other metals for improved characteristics. Ion beam coating orthopaedic parts by ion implanting the parts with zirconium ions while the parts are immersed in an oxygen-containing background gas is also disclosed. The adhesion of the graded interface zirconium oxide surface layer so formed is further improved by the initial removal of surface contamination using an ion bombardment and the deposition of an intermediate layer of platinum or similar metal or silicon between the orthopaedic metal component and the zirconium oxide. Furnace heating results in atomic interdiffusion to enhance adhesion between the surfaces. The zirconium oxide provides a low friction, low wear articulating surface. The graded interface may be characterized by a blackish color and a transition between pure zirconium oxide and pure intermediate layer that extends over a thickness of hundreds of Angstroms. In an alternative embodiment, the thickness of the zirconium oxide coating may be increased by also adding a simultaneous deposition of zirconium oxide on the parts.

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/589,018 filed Jan. 19, 1996, now U.S. Pat. No. 5,674,293.

TECHNICAL FIELD

The present invention relates to improved coating of orthopaedic implantcomponents.

BACKGROUND OF THE INVENTION

It is known that coating of the metal articulating surface oforthopaedic devices can improve wear and decrease friction, and improveother surface properties, in implantable orthopaedic devices. Forexample, it is possible to improve these properties by replacing themetal surface with a continuous surface layer consisting of a ceramicmaterial. Zirconium oxide (ZrO₂ or zirconium oxide) has been found to bebeneficial. This is described in U.S. Pat. No. 5,037,438, J. Davidson etal and also in an article titled "Low Wear Rate of UHMWPE AgainstZirconium oxide Ceramic (Y-PSZ) in Comparison to Alumina Ceramic andSS316L Alloy", found in the J. of Biomed. Mat. Res. 25, p. 813 (1991).

In the method of growing a zirconium oxide coating on orthopaedicdevices described in U.S. Pat. No. 5,037,438, the zirconium oxide isformed by diffusing oxygen gas into the metal at a high temperature in afurnace for a suitable period of time. The process occurs at atmosphericpressure and results in the chemical reaction of the zirconium metalworkpiece and oxygen to form a surface layer of zirconium oxide. Thismethod, however, requires that the prothesis be fabricated from purezirconium metal, which may not be suitable for implantation into humans.

One well-known alternative is the use of ion implantation of variouselements of the implant component for improving the wear, friction, andother surface properties of many metal alloys. See "Surface Modificationof Metals by Ion Beams", Elsevir Sequoia (1984). For alloys containingprimarily the elements cobalt and chromium, both ion implanted nitrogenand titanium have been shown to improve friction and wear properties.See "Friction and Wear Behavior of Cobalt-Based Alloy Implanted with Tior N", Mat. Res. Soc. Symp. Proc. 27, p. 637 (1984). For orthopaedicsurgical implants, cobalt-chrome alloy implanted with nitrogen has beenfound to improve the corrosion and subsequent wear/friction propertiesof the prosthetic joint. See "Medical Applications of Ion BeamProcesses", Nuc. Inst. and Meth. in Physics Res. B19/20., pg. 204-208(1987). Further, the idea of using an ion beam and physical vapordeposition simultaneously bombarding a metal surface has been in use forflat substrates. See "Properties of Aluminum Nitride Films by an IonBeam and Vapor Deposition Method", Nucl. Inst. and Meth. in Phys. Res.B39, p. 178 (1989).

Ion implantation of nitrogen does produce some improvement in the wearand friction properties of those alloys containing predominantly cobaltand chromium when rubbing against ultra-high molecular weightpolyethylene (UHMWPE) in a laboratory test, such as a pin-on-disk weartest. See U.S. Pat. No. 5,123,924, Sioshansi et al.

Zirconium ions have been ion implanted into iron and steel to improvethe corrosion properties. See "Surface Modification of Iron and Steel byZirconium or Yttrium Ion Implantation and Their ElectrochemicalProperties", from "Surface Modification of Metals by Ion Beams 7", Eds.F. A. Smidt, G. K. Hubler, and B. D. Sartwell, Elsevier Sequoia S. A.,p. 1 (1992).

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide improved coatingof orthopaedic prostheses and to provide improved orthopaedic prostheseswith a coating on the articulating surface.

It is a further object of this invention to provide a method forion-coating the articulating surface of femoral hip ball and kneeorthopaedic prostheses made from either an alloy primarily consisting ofcobalt and chromium or an alloy primarily consisting of titanium orstainless steel alloys.

It is a further object of this invention to provide such a method forcreating orthopaedic prostheses having a longer useful life by reducingpolyethylene wear debris.

It is a further object of this invention to improve the adhesion betweenthe zirconium oxide ion coating and the said femoral component todecrease the possibility of coating delamination or failure.

According to the present invention, a coating of platinum groupmaterials may be provided on the implant parts. The parts thus coatedare found to have improved surface characteristics such as wear.

In another embodiment of the invention, the implant parts are coatedwith a platinum group material or silicon and with zirconium oxide. Thismay be achieved by using a conventional method for coating the partswith the platinum group material or silicon and by implanting zirconiuminto the parts in the presence of oxygen, according to U.S. Pat. No.5,383,934, having common inventors and assigned to a common owner.

BRIEF DESCRIPTION OF DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings in which:

FIG. 1A illustrates part of an implant component coated with a platinumgroup material according to the present invention.

FIG. 1B illustrates part of an implant component coated with a platinumgroup material or silicon and zirconium oxide according to the presentinvention.

FIG. 2 illustrates the coating process of a preferred embodiment of thepresent invention.

FIG. 3 is a schematic diagram of an ion implantation apparatus foraccomplishing the method of this invention;

FIG. 4A is a detailed, partly cross-sectional view of a preferredembodiment of the apparatus of FIG. 3;

FIG. 4B is a top view of the device of FIG. 4A illustrating the closepacking of the parts being coated;

FIG. 5 illustrates a depth profile of a flat disk, demonstratinginterdiffusion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the description of the preferred embodiment relates to coating ofan implant component that is made of an alloy composed primarily ofcobalt and chromium, such as either case ASTM-F75 alloy or wroughtASTM-F799 alloy, the component may also be a surgical alloy composedprimarily of titanium, such as titanium-6aluminum-4vanadium. Thecomponent may also be made of a surgical grade of stainless steel. Thecomponent is preferably a femoral hip ball or femoral knee component.

Ion implantation of zirconium ions into orthopaedic prostheses that areimmersed in a low pressure, oxygen-containing gas will result in theaddition of zirconium atoms to the workpiece together with a concurrentchemical reaction with oxygen to form a low-friction surface layer ofzirconium oxide. A method of forming a highly adherent surface layer ofzirconium dioxide on a cobalt-chrome alloy substrate was disclosed inU.S. Pat. No. 5,383,934, which is fully incorporated herein byreference. This patent discloses ion implanting parts of implantcomponents with zirconium ions in the presence of an oxygen-containingbackground gas. The implanted zirconium reacts with the oxygen to formzirconium-oxide.

Zirconium metal and zirconium oxide do not, however, inherently tend tobind to alloys substantially consisting of cobalt and chromium(cobalt-chrome or cobalt-chromium alloy). This is a consequence of thelow mutual solubility and low chemical reactivity between zirconium andcobalt or chromium. In the method disclosed in U.S. Pat. No. 5,383,934,this deficiency is partially addressed by using the ion implantationtechnique to deposit atoms of the zirconium oxide coating below thesurface of the cobalt-chrome prosthesis. This is described as a gradedor blended interface, which avoids the abrupt transition in materialproperties typical of a coating that is deposited solely on the externalsurface by such common techniques as evaporation, sputtering or electronarc discharge.

One alternative is to use a platinum group material for coating part ofan implant component, which bonds well with metal implants. As used inthis specification, including the claims herein, the term "platinumgroup material" refers to platinum, iridium, ruthenium, rhodium andpalladium, as well as alloys thereof. In the preferred embodiment, theimplant parts are coated with substantially pure platinum, substantiallypure iridium, or a platinum--10% iridium alloy.

FIG. 1A illustrates a part of an implant component 1 that has beencoated according to one embodiment of the present invention. In thisembodiment, the implant component 1 is made of a cobalt-chrome alloy 3and is coated with a thin layer of a platinum group material 2.

The coating of a thin layer of a platinum group material on thecomponent can be performed by any of several well-known processes, suchas evaporation, sputtering, or arc discharge. The surface of theprosthesis should be pre-cleaned of impurities or oxides usingaccelerated ion bombardment, a well-known coating process step.

FIG. 2 illustrates the preferred method for coating parts of the implantcomponent 1. While the process is described in terms of the preferredembodiment, one of skill in the art would understand that variousmodifications in the process and the operating parameters for theprocess could be made, and are within the scope of the invention. Theprocess begins at step 8a of cleaning the part, by bombarding thecomponent with accelerated ions to remove contamination and oxides. Anyconvenient ion species may be used for this step. A commonly usedtechnique is glow discharge with argon ions. The step 8a is followed bya step 8b of depositing a thin layer of a platinum group material by anyof several common coating technologies, such as evaporation, sputteringor arc discharge. In the preferred embodiment, the step 8b is performedby sputtering achieved by bombarding a plate made of the platinum groupmaterial with two keV argon ions preferably using a commerciallyavailable magnetron sputtering apparatus, for example, one availablefrom AJA International, North Scituate, Mass. The coated prosthesis isthen heated at a step 8c in an oxygen-free furnace which may be providedeither by vacuum or an inert gas atmosphere, such as argon. The heat ispreferably between 100° C. and 1000° C. and lasts for at least tenminutes. In a preferred embodiment, the heating is for a period of oneto three hours at 750° C. The residual oxygen can be as high as 1×10⁻³Torr for platinum. This heating results in interdiffusion of the atomicspecies of the coating and the implant component. For a preferredembodiment, processing is complete following the step 8c.

The platinum group material coating is preferably 0.2 to 5 microns inthickness over the articulating surface. In a preferred embodiment, thecoating is about 1 micron thick.

FIG. 1B illustrates an alternative embodiment of the present invention.FIG. 1B shows a part of an implant component 5 made of cobalt-chrome 6and having both a platinum group material or silicon coating including azirconium graded interface layer 7 and an external zirconium oxidecoating 8. The use of a metal coating combined with a zirconium oxidecoating may be desirable because the mutual solubility or chemicalreactivity between zirconium and zirconium oxide with materials otherthan cobalt-chrome can be relatively high, thus permitting betteradhesion with the zirconium oxide than with simple zirconium oxidecoating or even a graded zirconium oxide interface alone. Such anintermediate material requires a common mutual solubility with both themetal of the implant component, such as a cobalt-chrome, and zirconium.Platinum group materials are acceptable intermediate materials sincecobalt and chromium are highly soluble in platinum group materials. SeeASM Handbook #3 for solubility diagrams. Zirconium has a greatersolubility in platinum group materials than in cobalt or chromium.Silicon is also an acceptable material. Silicon chemically reacts withcobalt and chromium to form metal suicides. Zirconium also reacts withsilicon to form a silicide. Adhesion is greatly improved by mutuallydiffusing the materials together at elevated temperature in anoxygen-free environment.

Thus, as explained in greater detail below, the component may be treatedby coating the articulating surface with a platinum group material orsilicon as an adhesion coating and by zirconium oxide achieved throughion implantation of zirconium in the presence of oxygen.

For ion implantation of zirconium, it is known to deposit the zirconiumatoms at some depth below the surface of the workpiece. Oxygen oroxygen-containing molecules will diffuse into the workpiece during theion bombardment. The oxygen molecules react chemically with the ionimplanted zirconium atoms to form an oxide. This process may beaccelerated by the energy provided by the bombarding ions. The zirconiumoxide molecules gradually increase in number with dose and eventuallymerge into a continuous layer at or above a preferred dose depending onion energy and angle of incidence. High energy ion bombardment ofsurfaces also results in the loss of surface atoms by a process calledsputtering. Sputtering removes both workpiece atoms and some of the ionimplanted zirconium atoms such that it is not always possible to achievea high enough concentration of zirconium to form pure zirconium oxide. Athin layer of nearly pure zirconium oxide will often form only when theangle of incidence of the zirconium ion beam is nearly normal to thesurface, because this condition results in the lowest rate ofsputtering.

The ion implantation process does not deposit every implanted atom atprecisely the same depth. A range of depths inevitably occurs. This isbeneficial for creating a graded or blended interface between theplatinum group material or silicon intermediate layer and zirconiumoxide surface coating which is not atomically abrupt but graduallyvaries in composition over a range of depths. Such a graded interfacepreferably varies smoothly between the composition of the pure workpiecematerial and nearly pure zirconium oxide at the surface and permits thephysical properties between the two materials to change gradually over arange of depths. Such a graded or blended interface is beneficial forproviding improved adhesion compared to conventional coatings, which aresimply laid on the surface of the workpiece and which may more easilydelaminate at the interface.

Referring again to FIG. 2, the process for coating component 5 isdescribed. While the process is described in terms of the preferredembodiment, one of skill in the art would understand that variousmodifications in the process and the operating parameters for theprocess could be made, and are within the scope of the invention. Theprocess may begin at the step 8a of cleaning the part, by bombarding thecomponent with accelerated ions to remove contamination and oxides. Anyconvenient ion species may be used for the step 8a. A commonly usedtechnique is glow discharge with argon ions. The process continues bythe step 8b of depositing a thin layer of the platinum group material orsilicon by any of several common coating technologies, such asevaporation, sputtering or arc discharge. In one embodiment, the coatingof the platinum group material or silicon is about 0.1 to 1 microns inthickness. In a preferred embodiment, the coating is 0.5 micron inthickness.

Optionally, the step 8e of coating the articulating surface of the piecewith a layer of zirconium is performed. In a preferred embodiment, thelayer is about 0.2 micron in thickness. This also may be done bymagnetron sputtering.

Following the step 8b, or in one embodiment step 8e, is the step 8cwhere the coated prosthesis is heated in an oxygen-free furnace whichmay be provided either by vacuum or an inert gas atmosphere, such asargon. The heating is preferably for a period of one to three hours at750° C. The residual oxygen can be as high as 1×10⁻³ Torr for platinumgroup materials and 1×10⁻⁵ Torr for silicon. This heating results ininterdiffusion of the atomic species of the coating and prosthesis.

Following the step 8c is a step 8d where the component is exposed to azirconium ion beam in an oxygen-containing gas, such gases typicallyincluding at least one of oxygen, ozone, water vapor, or hydrogenperoxide, and providing an ion beam consisting primarily of zirconiumions to the immersed component to form an implanted zirconium oxidelayer in the component. The partial pressure of the oxygen-containinggas or gases should be in the range from 5×10⁻⁶ Torr to 1×10⁻³ Torr andpreferably at 4×10⁻⁵ Torr. The oxygen-containing gas may be composed ofany one or more of the compounds of pure oxygen, water vapor, ozone, orhydrogen peroxide. Other gases, such as nitrogen, may also be presentand which do not contribute to the formation of the oxide. Theoxygen-containing gas is preferably oxygen.

The process may further include providing simultaneously a source ofzirconium, which may include zirconium or zirconium oxide. This can bedone using any of a variety of conventional techniques. A flux ofzirconium may be supplied by evaporation using an electron beamevaporator. In a preferred embodiment, the ion beam and the flux ofzirconium treat similar surface regions of the immersed component duringeach complete cycle of movement of the component relative to the ionbeam.

The zirconium ion beam preferably has an energy of between 20 keV and400 keV. The zirconium ion beam preferably delivers a total ion dose ofbetween 5×10¹⁶ and 5×10¹⁸ ions/cm². Preferably, for a flat surface whosenormal axis coincides with the direction of the ion beam, the dose is3×10¹⁷ atoms/cm² for a 150 keV zirconium ion beam. Curved or tiltedsurfaces may require other preferred doses, depending on the geometry.An appropriate dose will convert a layer to substantially all zirconiumoxide, and can be determined by one of ordinary skill in the art viaroutine experimentation.

The process may further include maintaining the immersed componentequilibrium temperature in the ion beam at a temperature between 25° C.to 600° C., which may be accomplished by adjusting the cooling to thefixture to which the component is mounted during treatment. A preferredtemperature is 250° C.

The process may further include simultaneously moving the componentrelative to the ion beam about two transverse axes during implantationto create a more uniform implanted layer. The ion beam may be providedat an angle to the component. During the formation of the gradedzirconium oxide interface layer, the process may further includeselection of the deposition rate per unit area for the zirconium oxidesurface coating which closely equals the rate of loss caused byzirconium ion beam sputtering. For an immersed component which exhibitsdifferent average sputtering rates for different regions of its surface,the relative flux per unit area is preferably first selected to form anion implanted zirconium oxide graded interface layer in a region oflowest average sputtering followed sequentially by selection of relativeflux for regions of higher average sputtering.

The implanted zirconium oxide graded layer is preferably from 50 to 5000Angstroms thick.

Many useful components have curved or tilted surfaces that cannot form anearly pure ion implanted buried layer of zirconium oxide because of ahigh rate of sputtering by the ion beam due to the angle of incidence ofthe beam at the surface. This limitation may be overcome by additionallyproviding a source of zirconium or zirconium oxide which deposits acoating on the workpiece at a rate which closely equals the rate of lossof atoms caused by sputtering. The flux of zirconium may be provided byknown methods such as high temperature evaporation, sputtering, orelectron arc discharged. This coating may be removed by sputtering asrapidly as it is deposited, providing no substantial net gain or loss ofeither the coating or workpiece materials. Such a coating rate allowsatoms of the zirconium ion beam to be nearly completely retained belowthe surface of the workpiece without losses caused by sputtering. Theconcentration of ion implanted zirconium atoms below the surface canthen be increased to a preferred level by selecting a dose which makes azirconium oxide graded interface layer. A preferred rate of coatingdeposition depends on geometry of the work piece and may be determinedvisually by a blackening of the surface after a sufficient zirconium iondose is provided to form the zirconium oxide graded interface layer. Aninsufficient coating rate during the ion implantation of zirconiumcauses a silver-grey color on a polished surface, and an excessive ratecauses a colored hue, such as red or green, which is characteristic of areflection optical interference effect.

There is shown schematically in FIG. 3 ion implantation apparatus 10 forextremely uniformly ion beam implanting or coating irregularly-shapedparts. Apparatus 10 includes rotatable turntable or fixture 12 fixed torotatable shaft 14. Below turntable 12 there is a fixed disc 18 having agear-engaging surface to act as a fixed sun gear. Parts 22, 22a to becoated, for example prosthetic hip balls, are mounted on shafts 20, 20a,respectively, to which are fixed planetary gears 24, 24a, that areengaged with the gear engaging surface of sun gear 18. When shaft 14 isrotated in the direction of arrow 15, balls 22 are caused to rotatearound axis 15 as well as shaft axis 23, 23a to simultaneously rotateparts 22, 22a about two transverse axes. Preferably the angle betweenaxis 15 and axis 23 is acute, and an angle of 49° has been found toresult in extremely uniform ion treating of the surfaces of parts 22,22a.

While the parts are rotating, they are exposed to one or more ion beams30 and 32 that are preferably provided at a slight angle to plan 34 offixture 12 so that the parts do not shadow each other. For implantationof prosthetic hip ball components, the beam axis is preferablyapproximately 3° to 10° from plane 34. The important parameter is theprevention of shadowing of the part by another. The beam angle necessaryto accomplish this may be determined by drawing a line from the bottomof part 22 to the top of part 22a and determining the angle at whichthat line intersects plan 34. When the beam is provided at least thisangle, the part closest to the beam source will not shadow the rearpart.

Apparatus 50, FIG. 4A, has been successfully used for uniformly ion-beamcoating prosthetic hip balls with zirconium ions on cobalt chromiumalloy prostheses. Preferably, the balls are spaced as closely aspossible together so that ion beam is not wasted.

Beams 53, 55, FIG. 4A, are typically approximately one inch in diameterand are preferably scanned in relation to the parts being coated byeither translating fixture 60 in the direction of arrow 16, FIG. 3, orelectrostatically deflecting the beams for example by using deflectionmechanism 82, FIG. 4A, including plates 82a, 82b for applying a voltagegradient across the beam. Preferably, the part-holding fixture iscontinuously translated up and down a distance approximately equal tothe height of the parts being coated to insure that the beams areuniformly scanned across the surfaces being coated.

The ion beam dose may be chosen as desired, and is preferably betweenabout 5×10¹⁶ and 5×10¹⁸ ions/cm². The ion beam current density isdefined as the ion beam current divided by the cross-sectional areawhose normal axis is parallel to that of the direction of the ion beam.The ion beam current density is typically chosen to be betweenapproximately 200 to 2,000 microamperes/cm² with a beam energy betweenabout 20 keV and 400 keV. The ion beam current density is chosen as highas possible consistent with the ion beam generation equipment used so asto provide a high speed economically viable process. The total ion beampower is defined as the ion beam current times the accelerating voltageapplied to the beam. The total ion beam power divided by the total areaswept out by the array of workpieces defines the ion beam powerdissipation density. The ion beam power dissipation density is selectedto maintain the workpieces at an average temperature between 50° C. to600° C. during processing.

A spherical workpiece, such as a femoral hip prosthesis, can be ion beamimplanted with a graded interface of zirconium oxide using the apparatusof FIGS. 4A and 4B. It is realized that with said apparatus, only alimited area on a spherical workpiece can be ion beam treated to producea graded interface of ion implanted zirconium oxide when a fixed fluxper unit area of zirconium oxide coating deposition and a fixed flux perunit area of zirconium ion beam is applied. It is further realized thatsuch a limited area consists of those regions on the spherical workpiecesubjected to a similar average rate of sputtering as the sphericalworkpiece is manipulated to present all of its articulating surface tothe zirconium ion beam and coating deposition. It is further realizedthat once a blackish graded interface zirconium oxide layer has formed,an increase in the coating deposition flux per unit area relative to thezirconium ion beam flux per unit area will deposit a zirconium oxidecoating over the blackish graded interface zirconium oxide layer.

Therefore, an ion implanted graded interface zirconium oxide layer canbe introduced into a spherical workpiece by simultaneously subjectingthe surface to a zirconium oxide coating deposition flux per unit areaand a zirconium ion beam flux per unit area while immersed in a partialpressure of oxygen. The total dose of zirconium ions and the fluxes perunit area are first selected to form a blackish graded interface layerin a region on the workpiece which is subjected to the lowest averagerate of sputtering. If the apparatus of FIGS. 4A and 4B is employed,this region will be on the rotation axis of the spherical workpiece.After the first region is so treated, the coating deposition flux perunit area relative to the zirconium ion beam is incrementally increasedto form the blackish graded interface layer in a region subjected to ahigher average rate of sputtering. This process is repeated until allthe desired surface regions are so treated. On a spherical workpiece acoating of zirconium oxide will accumulate on those regions that havesmaller average rates of sputtering than the region currently beingtreated to form the blackish graded interface layer of ion implantedzirconium oxide.

It is realized that the method of forming a blackish ion implanted layerof zirconium oxide described for spherical orthopaedic workpieces mayalso be applied as a general method for other shapes of orthopaedicworkpieces, such as femoral knee prostheses. It is further realized thatother shapes of orthopaedic workpieces may require a different type ofapparatus for manipulating the workpiece to ensure uniform treatmentthan the type shown in FIGS. 4A and 4B.

A preferred embodiment for ion treating a CoCr spherical femoral hipprosthesis component includes the steps of coating the articulatingsurface with about 0.5 micron thickness of platinum as described above,depositing about a 0.2 micron thickness of zirconium on the platinumcoating, heating the component at 750° C. for about three hours in ahigh vacuum (1×10⁻⁶ Torr), and implanting zirconia under the followingprocess

    ______________________________________                                        (a)  Blending Phase                                                              Ion Beam: Zirconium 90                                                        Beam Current: 0.5 mA                                                          Beam Energy: 190 keV                                                          Scanned Area: 58 cm                                                                           .sup.2                                                        Evaporant: Zirconium                                                          Deposition Rate: 0.4 to 10Å/sec in 3 steps (determined to                  balance sputter rate)                                                        Oxygen Pressure: 3 × 10.sup.-5  Torr                                    Dose at Each Step: 2 × 10.sup.17  Zr.sup.+ /cm.sup.2                    Rotational Axis 45° to axis of ion beam and evaporator                 of Prosthesis:                                                               (b) Growth Phase                                                               Ion Beam: Zirconium 90                                                        Beam Current: 200 μA (constant)                                            Beam Energy: 36 keV (constant)                                                Scanned Area: 58 cm.sup.2                                                     Evaporant: Zirconium                                                          Deposition Rate: 1.40Å/sec. (constant)                                    Oxygen Pressure: 3 × 10.sup.-5  Torr                                    Time Duration: 2 Hours                                                     ______________________________________                                    

A test was performed to measure intermixing of the platinum andzirconium layers on a cobalt-chrome alloy, without ion implantation ofzirconium into the workpiece. A flat CoCr alloy disk specimen was firstcoated with 2000 Angstrom of platinum and then coated with 3000 Angstromof zirconium using ion beam sputtering. The specimen was then heated ina vacuum furnace for 3 hours at 750° C.

FIG. 5 shows a depth profile of the surface of the sample using Augerelectron spectroscopy. As can be seen, the platinum coating has beenpenetrated on the left side with zirconium and on the right side by boththe cobalt and chromium of the substrate alloy. The specimen processedby the above process now has enhanced adhesion between the zirconiumouter coating and the CoCr substrate through the use of the intermediateplatinum layer.

Although specific features of the invention are shown in some drawingsand not others, this is for convenience only as some feature may becombined with any or all of the other features in accordance with theinvention. While the invention has been disclosed in connection with thepreferred embodiments shown and described in detail, variousmodifications and improvements thereon will become readily apparent tothose skilled in the art. Accordingly, the spirit and scope of thepresent invention is to be limited only by the following claims.

What is claimed is:
 1. A process for treating an articulating surface ofa metallic femoral component of an orthopaedic implant, comprising stepsof:(a) depositing on the articulating surface of the component anadhesion coating of a platinum group metal selected from the groupconsisting of platinum, iridium, ruthenium, rhodium, palladium andalloys thereof; (b) heating the component under conditions sufficient tocause atomic interdiffusion of the adhesion coating into the metalliccomponent thereby forming a first graded interface between thearticulating surface and the adhesion coating; (c) immersing thecomponent in an oxygen-containing gas; and (d) exposing the immersedcomponent to an ion beam of zirconium under conditions sufficient toform a zirconium oxide coating on said adhesion coating.
 2. The processof claim 1, further comprising a step of:simultaneously with step (d),exposing the immersed component to a flux of zirconium atoms.
 3. Theprocess of claim wherein:step (b) comprises heating the metallic femoralcomponent at a temperature between about 100° C. and 1000° C. for atleast 10 minutes.
 4. The process of claim 1, wherein the partialpressure of said oxygen-containing gas is in a range from 5×10⁻⁶ to1×10⁻³ Torr.
 5. A metal femoral component produced by the process ofclaim
 1. 6. The process of claim 1 wherein the adhesion coating isselected from the group consisting of platinum, iridium and aplatinum-iridium alloy.